Ilmenite Upgrading by the MURSO Process
H.N. Sinha (Research Manager/ Australia)
As published in : "Light Metals 1972", AIME-TMS Publications,
New York, 1972.
A process to upgrade ilmenite to +95% TiO2, i.e. rutile grade product is described. The process consists of first oxidizing substantially all the iron values associated with titanium dioxide to the ferric state (Fe2O3), then reducing the ferric iron to ferrous state (FeO) to produce a "synthetic ilmenite" material which is more reactive than natural ilmenite and amenable to leaching with 20% HCl at 108-110oC. The process is capable of upgrading ilmenite from rock as well as alluvial (sand) deposits, and it permits the ready removal of chromite and other such ferruginous impurities with negligible loss of titanium oxide. It has no effluent disposal problems and incorporates well established chemical engineering technology.
Tests have shown that the Murso process can be applied successfully to all commercial ilmenites whether they be in the form of beach sands or rock deposits. The process is such that where ferruginous impurities such as chromite occur in association with ilmenite they can be removed readily without any significant loss of TiO2 values.
In the main, commercial ilmenites contain 52-55% TiO2 combined with 40-45% iron oxides. In upgrading ilmenite, iron oxides ( and oxides of manganese and magnesium which substitute for iron oxides in the ilmenite lattice) have to be removed if the required quality is to be obtained.
The Murso Process
The flowsheet of the Murso process is given in Fig.1. There are five major steps in the process:
3. Leaching of the reduced product.
4. Solid-Liquid separation.
5. Recovery and recycling of the hydrochloric acid from ferrous chloride liquors.
The iron in ilmenite is present in both the ferrous and ferric states and the ratio of ferrous to ferric iron varies in different ilmenites and depends upon the degree of alteration of the mineral. In the Murso process substantially all the iron values associated with titanium dioxide in the mineral are oxidized to the ferric state. This places ilmenites from different localities (and types) and having differing ferrous to ferric ratios, on a common footing and the process is capable of treating all ilmenites irrespective of their origin.
Both oxidation and reduction steps affect the grade of the product and a high grade (³ 95%) TiO2 is to result. Oxidation is an exothermic reaction as shown below :
Temperature of Oxidation
Although the oxidation of ilmenite starts at temperatures as low as 250oC, the rate at such low temperature is very slow and does not become appreciable until a temperature of 800oC is attained. The oxidation rate increases with rise in temperature.
In practice, however, the choice of oxidation temperature is determined by sintering temperature of ilmenite, fuel economy, and materials of construction for the reactor. While the sintering charachteristics of ilmenites are dependent on their chemical composition, most ilmenites start to sinter above 1000oC. Since sintering would decrease the rate and extent of oxidation, sub-sintering temperatures are preferred. Furthermore, by operating below 1000oC materials problems as well as heat requirements are reduced.
In view of the above factors, the oxidation step is carried out at a temperature of 900-950oC. Since the heat of reaction is not sufficient to maintain the ilmenite at the oxidation temperature, the heating of the charge is achieved by fuel injection in the bed with an excess of air to give approximately 10% oxygen in the products of combustion. The partial pressure of oxygen required for oxidation is very low and the experimental results show that the rates of oxidation of ilmenite in air and a gas containing 10% oxygen are comparable with each other.
In commercial operations, oxidation is no economic burden since the material would in any case have to be heated to the reduction temperature (which, in the case of Murso process is carried out under controlled conditions of temperature and partial pressure of oxygen).
Fluid Bed Reactor
Since ilmenite produced from beach and deposits occurs as a closely sized material (size range between 100 to 200 microns), it is an ideal material for fluidization. Thus the steps of oxidation and reduction are preferably carried out in fluid bed reactors. In continuous operation, both oxidation and reduction will incorporate two stages to minimize short circuiting of particles from fluid beds as well as to obtain a maximum utilization of reacting gases particularly the reductant. In oxidation, the first stage also serves the function of pre-heating the charge.
Preferably the reduction is carried out in a fluid bed at temperature of 850-900oC (the process being so controlled that the reduction from ferric to ferrous state occurs with a minimum formation of metallic iron). This extends further the process of formation of a large number of sub-grains in the original ilmenite grain which starts at the oxidation stage and produces a very reactive "synthetic ilmenite" product. The x-ray diffraction pattern of the product is similar to original ilmenite but the micro-structure is quite different and consists of a large number of small grains.
The reduction of oxidized ilmenite with hydrogen may be represented by the following reactions :
The calculated values for the heat of reduction of oxidized ilmenite show that the partial reduction i.e. ferric to ferrous state, is slightly exothermic whereas reduction of ilmenite to metallic iron is endothermic. D.T.A (Differential thermal analysis) experiments have confirmed the exothermic nature of the reduction reaction as used in the Murso process. This is a very important consideration when designing large scale fluid beds as the question of providing heat for endothermic processes becomes quite critical in fluid beds especially for reactions where strongly reducing conditions are required. While heat could be supplied to the bed by heating the incoming reductant gas, if this is carried out beyond a certain temperature (say above 850oC), limitations are imposed on the materials forming the hearth of the reactor. However, where the reaction is exothermic, as is the case above, very little preheating of the reductant gas is required. The solids from the oxidizer enter the reducer at a temperature selected for the reduction step.The heat required to bring the reductant to the reaction temperature is derived from the exothermicity of the reduction reaction itself. Any heat deficit can be supplied readily by by slight super heating at the oxidation step.
In those processes necessitating the complete reduction of iron to the metallic state, temperature over 1000oC are required for kinetic reasons and even at these temperatures the reaction is thermodynamically very unfavourable. The oxidation step in the Murso process and limiting the reduction to ferric-ferrous transformation creates very favourable equilibrium and kinetic conditions.
In view of favourable equilibrium conditions and kinetics for partial reduction of oxidized ilmenite, the choice of a reductant in the Murso process depends upon economic rather than chemical considerations. Since fluid bed reactors have significant economic advantages ( e.g. lower capital cost, better heat and mass transfer) over other reactors such as rotary kiln or multiple hearth furnace, gaseous reductants are preferred. However, solid reductants are equqlly effective as far as the chemistry of the process is concerned. Amongst gaseous reductants, hydrogen is preferred because the rate of reduction below 9000C is much higher than with carbon monoxide. High purity hydrogen is not necessary and tests have shown that a gas mixture consisting of hydrogen, carbon monoxide, carbon dioxide and some water vapour is quite effective. This type of gas compositon may be economically produced by the steam reforming of naphtha or natural gas, a typical reductant gas composition being 70% H2, 13% CO2 and 4% water vapour.
As is the case in the oxidation step, the rate of reduction increases with increase in temperature but does not become appreciable till a temperature of 700oC is reached. In a commercial process, however, the temperature of reduction to a certain extent is dependant upon the temperature of material being discharged from the oxidizer and reduction temperature 50-100oC below the oxidation temperature is selected. A typical temperature of reduction is 850oC.
Leaching natural ilmenite in hydrochloric acid has been proposed but this involves the use of high acid concentrations and temperatures so that leaching operations have to be carried out under pressure and in two stages. The reactivity of naturally occurring titaniferrous ores is so poor that even under above conditions the time required for adequate leaching is excessive. The design and the materials of construction for the leaching vessels also present considerable problems and efficient recovery of 32% HCl from chloride containing effluents by existing commercial processes is considerably more difficult and expensive than is the recovery 0f 20% HCl.
The purpose of the leaching is to dissolve selectively iron oxide from ilmenite lattice with a minimum loss of titanium values. However, some titanium does go into solution and a certain amount of it is subsequently precipitated from the solution as a fine material (< 75 microns). The main factors which affect the rate of leaching, titanium dissolution and its hydrolysis are acid concentration and leaching temperature. Both high acid concentration and high temperature favour high rate of leaching. But other factors such as titanium loss in solution, production of fines, materials of construction for leaching vessels and economic recovery of hydrochloric acid influence the choice of acid strength and temperature of leaching.
Taking the above points into consideration, the optimum conditions for leaching in the Murso processwere found to be 20% HCl and temperature of 108-110oC at atmospheric pressure. In practice, a 20% excess of 20% HCl over the stoichiometric amount required for leaching iron is used. Under these conditions, the kinetics of leaching are quite favourable so that batch-wise leaching is complete in 3-4 hours. This is attributed to the formation of "synthetic ilmenite" structure containing many lattice defects, mainly sub-grain boundaries which greatly enhance the process of diffusion which seems to control the leaching step. The amount of fines produced is 4-5% and titanium in leach liquors is less than 1% of the titanium input.
Oxides of manganese, magnesium, and vanadium which are structurally associated with TiO2 in the ilmenite lattice are effectively leached out with iron and partial leaching of aluminium oxide occurs also. The following table gives typical analyses of ilmenite, murutile, and commercial rutile.
Analyses of Ilmenite, Murutile, and Rutile
Heat of leaching
The heat associated with the leaching of iron oxide from reduced ilmenite by 20% HCl at 108oC according to the following reaction has been determined calorimetrically
The overall heat reaction up to 70-80% extraction was measured to be -21.3 Kcal/gm atom of iron dissolved. This was in agreement with an estimate of -22 ± 3 Kcal at 25oC using published thermodynamic data.
Since the leachinf reaction is exothermic, no external heating is required to maintain the system at the leaching temperature in well insulated vessels.
The leached solids are classified into a coarse and fine fraction using conventional equipment (classifier, filter and thickener). The coarse fraction has a grain size similar to the feed ilmenite. It is washed, filtered and calcined at 450oC to remove residual traces of moisture, iron chloride and hydrochloric acid. The fine fraction is also washed and, preferably, spray dried to the solid state. This product is suitable for use in the welding electrode industry.
Regeneration of Hydrochloric Acid
present day conditions, the problem of effluent disposal is vital in
establishing a commercial process. In fact one of the reasons for the change
from the sulphate to the chloride route for producing
TiO2 pigment is the cost and the difficulties associated with the
disposal of ferrous sulphate. The Murso
process has taken into account the important environmental pollution problem
and this one reason for incorporating the step of recovering hydrochloric acid
from ferrous chloride liquors. Ferrous chloride (including manganese, magnesium,
and titanium chlorides) can be easily hydrolysed, and
commercial processes are in operation for the recovery of hydrochloric acid. In
tha main, the steel pickling industry has been
responsible for the development of these processes but they can be easily
extended to ilmenite upgrading system as the problems
are very similar. The regeneration step produces 19-20% HCl
which is recycled and solid iron oxide which may may be used as a feed material in the iron industry,
thus avoiding any effluent disposal problems. The predominant reaction is :
The predominant reaction is :
Most of other metal chlorides present in the spent leach liquor (except calcium) behave similarly. Excellent recoveries are achieved but a minor amount of make-up acid is required.
Besides pollution problems, recovery of hydrochloric acid involves economic considerations also, since under most circumstances it would be cheaper to regenerate hydrochloric acid than to buy an equivalent amount of acid and then dispose of the ferrous chloride liquor. It has been estimated by one equipment manufacturer that the cost of regeneration is approximately £5 per ton (on a 100% HCl basis) including make up acid cost. The cost includes capital and operating charges.
Upgrading Chrome Contaminated Ilmenites
Many ilmenite deposits are associated with the mineral chromite. Complete separation of chromite from ilmenite by magnetic separation is very difficult because of the similarity in their magnetic susceptibilities. In both the sulphate acid and the chlorination processes of pigment manufacture, the presence of relatively small quantities of chromium mineral in association with the titaniferrous ores is undesirable. At present ilmenite containing more than 0.1% Cr2O3 is unacceptable by the sulphate pigment industry and a process to treat ilmenites containing more than the acceptable Cr2O3 content would be of great advantage as large tonnage of this material are currently being dumped.
Murso process is the only process so far reported which provide a method of converting this hitherto unusable ilmenite into rutile grade product. Because of the selective nature of the process chromite and other ferruginous minerals associated with ilmenite remain virtually un attacked during processing. The clacined product consists therefore of non-magnetic grains of upgrade together with magnetic grains of chromite (and other ferruginous minerals) which can be separated from each other with only insignificant losses of titanium values by simple magnetic separation.
The Murso process yields an upgrade containing more than 96% TiO2 and about 0.1% Cr2O3 from an ilmenite, containing as high as 5.5% Cr2O3 with the loss of as little as 1.5% of the TiO2 values.
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